As we have remarked previously, civility is the greatest gift one can bestow on the creationist conmen, the right-wing liars, and the religious bigots--not to mention the hordes of ignorant blowhards in the blogosphere. To treat their positions with civility is to already legitimate them. The consequence of doing so is now available for all the world to see: the intellectually and morally depraved state of public culture in America today.
Volokh, by contrast, advocates the following standard:
[A]ll arguments -- good or bad, bigoted or saintly -- should be framed without the use of personal insults or invective, since such devices are not only offensive and likely to lead to a worsening of the discussion, but also unnecessary. Offensive ideas, fine; offensive modes of expressing those ideas, unnecessary and thus not fine. Cohen v. California is a good guide for constraining government repression of speech. I don't think it's a necessary or wise rule for editorial decisions in publications such as this one.
Finally, as I've noted before, let me preemptively respond to the near certain response: "But would you let commenters advocate the Holocaust? Slavery? Mass rape?" Well, though I support same-sex marriage, we need to have some sense of perspective: The denial of equal rights here ain't the Holocaust. There may well be a pale that certain things are beyond. Yet the pale should be pretty far out, and the fact that advocacy of mass murder is beyond it doesn't tell us much about condemnation of homosexuality.
Incidentally, my position is almost identical to Volokh's. I too conceded an exception for genocide, and as predicted, Leiter pounced. And I basically made the same response as Volokh--there is a qualitative difference between opinions we disagree with, even detest, and advocating mass murder.